A non-Newtonian fluid quasi-solid electrolyte designed for long life and high safety Li-O2 batteries

The Li dendrite growth and the liquid electrolyte volatilization under semi-open architecture are intrinsic issues for Li-O2 battery. In this work, we propose a non-Newtonian fluid quasi-solid electrolyte (NNFQSE) SiO2-SO3Li/PVDF-HFP, which has both shear-thinning and shear-thickening properties. The component interactions among the sulfonated silica nanoparticles, liquid electrolyte, and polymer network are beneficial for decent Li+ conductivity and high liquid electrolyte retention without volatilization. Furthermore, NNFQSE exhibits shear-thinning property to eliminate the stress of dendrite growth during repeated cycling. Meanwhile, when the force suddenly increases, such as a high current rate, the NNFQSE may dynamically turn shear-thickening to respond and mechanically stiffen to inhibit the lithium dendrite penetration. By coupling with the NNFQSE, the lithium symmetrical battery can run over 2000 h under 1 mA cm−2 at room temperature, and the quasi-solid Li-O2 battery actualizes long life above 5000 h at 100 mA g−1.

A novel non-Newtonian fluid quasi-solid electrolyte (NNFQSE) SiO2-SO3Li/PVDF-HFP is proposed in this work to address the major issue of anode (Li dendrite growth) and the inherent issue of Li-O2 battery as its semi-open structure (volatilization and leakage of liquid electrolytes). As a result, the lithium symmetrical battery can operate for more than 2000 hours at 1 mA cm-2 at room temperature, while the quasi-solid Li-O2 battery has a superior long life of more than 0.5 years (> 5000 h at 100 mA g-1). This is a very shocking result in the field of lithium-air batteries, especially in terms of stability. This suggests new ideas for the design of long-life lithium-air batteries. However, there are some issues in this work that require further discussion.
(1)What causes such high Li+ conductivity at room temperature? As presented, the 5.4×10-3 S/cm of quasi-solid electrolyte is higher than that of liquid electrolyte, such as, LiTFSI/TEGDME; (2)The physical properties of the quasi-solid electrolyte, especially its shear thickened property should be discussed. And the inhibition mechanism and inhibition effect of lithium dendrites should be clarified; (3)The residue of solid-state products in cathode and the pulverization and dendrite situation of lithium anode after long-term stability tests should be further discussed.
Reviewer #2 (Remarks to the Author): In this work, the authors reported that using SiO2-SO3Li/PVDF-HFP as non-Newtonian fluid quasisolid electrolyte in Li-O2 battery can effectively eliminate dendrite issues during cycling measurements along with good liquid electrolyte retention and Li+ conductivity. As the results, the battery can perform excellent cycling stability of 5000 hours at 100 mA g-1. Some issues are addressed as follows: 1.It is really hard to see that the S are uniformly distributed on SiO2-SO3Li particles from EDSmapping result in Figure S3. Please check the result and give more clear demonstration.
2.The FTIR results should give more clarifications between the SiO2, SiO2-SO3H and SiO2-SO3Li to prove the varied surface chemistry. It is claimed by author that the peak of Si-OH in 2347 cm-1 disappears in SiO2-SO3Li compared to SiO2 due to sulfonated treatment, but why there is a peak of Si-OH at 1632 cm-1 in both SiO2 and SiO2-SO3Li samples in Figure 2d and FigureS5? What the difference between these vibration features?
3.Please explain why the core level XPS spectra of S 2p seems very poor signal but showed much stronger intensity in survey spectra in Figure 2e.
4.The porosity of control-group samples should be provided to prove the superior microstructure properties of skeleton matrix materials. 5.Please explain why the stable electrochemical window of SiO2-SO3Li/PVDF-HFP is much higher than the others? Besides, LSV with same scan rate to low potential area should also provided to examine its reduction behavior.
6.The sample of SiO2 /PVDF-HFP showed relatively lower voltage hysteresis compared to SiO2-SO3Li/PVDF-HFP. In Figure 4b,c If reaction kinetics of Li+ in SiO2-SO3Li/PVDF-HFP is higher than other, why this phenomenon occurred?
Reviewer #3 (Remarks to the Author): This work reported a composite quasi-solid electrolyte SiO2-SO3Li/PVDF-HFP with non-Newtonian fluid properties for Li-O2 batteries (LOBs). Intensive mechanical and electrochemical characterizations were conducted to investigate the properties of the designed electrolyte. The assembled LOBs exhibited an ultra-long cycling life. But the current manuscript lacks analyses and discussions regarding battery chemistry and anode/electrolyte/cathode interfaces. The promotion effect was oversimplified by component interactions. There are also some problems to be clarified before the final acceptance. 1. The author calculated the contact angle of SiO2-SO3Li/PVDF-HFP with a different tangent center from control samples ( Figure S7). A wrong conclusion about wettability was obtained. From the direct view, the contact angles of the three samples are similar. The enhanced wettability of SiO2-SO3Li/PVDF-HFP is doubting. 2. The order of Figures in supporting information is inconsistent with the manuscript. 3. The preparation procedure of the cathode should be illustrated in the experimental section. 4. From presented photos of SiO2-SO3Li/PVDF-HFP before and after cycling, the cycled electrode became thicker than the initial, and why? 5. From a recent literature review of LOBs (doi.org/10.1039/D2CS00003B), the battery chemistry is significantly affected by the electrolyte. How the SiO2-SO3Li/PVDF-HFP electrolyte affects the growth and decomposition of discharge products is yet lacking, as well as the crystal structure and morphology. 6. Although SiO2-SO3Li/PVDF-HFP exhibited a wide electrochemical window, reactive oxygen species generated during the discharging and charging of LOBs could also induce the decomposition of the electrolyte and functional groups, especially during ultra-long operation. Relevant discussions and analyses are recommended in order to obtain a more comprehensive understanding of the stability of the designed electrolyte. 7. As shown in Figure S14, the impedance of the cycled LOB increased significantly. This could be caused by the coverage of undecomposed discharge products or the degradation/evaporation of the electrolyte. The actual reasons should be clarified. 8. From Figure 2c and Figure S2c, similar morphology was observed on SiO2-SO3Li and SiO2. How did the author define the "surface channels" and conclude the "deeper and richer" surface channels of SiO2-SO3Li than SiO2? 9. The introduction of the -SO3Li group significantly changed the morphology of composite polymers, enabling uniform distribution of SiO2 and providing Li+ diffusion channels. But the extra roles of the -SO3Li groups in promoting battery performance are ambiguous and lack compelling evidence.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): A novel non-Newtonian fluid quasi-solid electrolyte (NNFQSE) SiO2-SO3Li/PVDF-HFP is proposed in this work to address the major issue of anode (Li dendrite growth) and the inherent issue of Li-O2 battery as its semi-open structure (volatilization and leakage of liquid electrolytes). As a result, the lithium symmetrical battery can operate for more than 2000 hours at 1 mA cm -2 at room temperature, while the quasi-solid Li-O2 battery has a superior long life of more than 0.5 years (> 5000 h at 100 mA g -1 ). This is a very shocking result in the field of lithium-air batteries, especially in terms of stability.
This suggests new ideas for the design of long-life lithium-air batteries. However, there are some issues in this work that require further discussion.

Response:
We thank the reviewer for the positive comments on the impact of our work. We have revised the manuscript and answered questions following your comments.
1. What causes such high Li + conductivity at room temperature? As presented, the 5.4×10 -3 S/cm of quasi-solid electrolyte is higher than that of liquid electrolyte, such as, LiTFSI/TEGDME.

Response:
Thanks for the valuable comment. The high Li + conductivity of quasi-solid electrolyte SiO2-SO3Li/PVDF-HFP at room temperature can be mainly attributed to the plenty of liquid electrolyte LiTFSI/TEGDME inside and Li + -conductive groups -SO3Li.

Updates to the revised manuscript and supplementary information:
We have added these detail discussions in the revised supplementary information (supplementary Note 1, page S7) and mentioned in the revised manuscript on Page 12, highlighted in yellow.
2. The physical properties of the quasi-solid electrolyte, especially its shear thickened property should be discussed. And the inhibition mechanism and inhibition effect of lithium dendrites should be clarified; Response: Thanks for the valuable suggestion. As shown in Fig. 2f-g, the quasi-solid electrolyte exhibits a double shear-thickening behavior at initial shear rates and high shear rates respectively, and its intermediate state behaves shear-thinning. The special rheological properties reveal that the hydrogen-bonding between Si-OH and PVDF-HFP will be broken under the force slowly applied, which leads to the loss modulus (G'') become larger than storage modulus (G'), displaying shearthinning behavior. When subjected to a high-speed impact external force, the nanoparticles SiO2-SO3Li Editorial Note: Figure below reproduced from Qiao, L., Oteo, U., Martinez-Ibañez, M. et al. Stable non-corrosive sulfonimide salt for 4-V-class lithium metal batteries. Nat. Mater. 21, 455-462 (2022), with permission from Springer Nature.
To clarified the inhibition mechanism and inhibition effect of lithium dendrites, we have carried out the COMSOL Multiphysics. On the one hand, when the NNFQSE is in the state of shear-thinning, its mechanical stiffness will become more and more soft during the charging/discharging cycles, which effectively eliminates the stress caused by the develop of dendrite (see Fig. 5h-i). On the other hand, in areas where the lithium deposition is uneven, or the applied current density is of a large magnitude, the sudden increase of contact stress will make the quasi-solid electrolyte turn to shear-thickening. At such state, the stiffened NNFQSE can hardly be deformed even forced by aggressive stress, and thus suppress the dendrite growth mechanically. As can be seen in Fig. R2, with a same violent applied current density and a same initial protrusion on anode, the dendrite with the ristriction of the stiffened NNFQSE increased from 3.5 μm, to 3.9 μm, within 300 s, the growth rate is only about 11% (Fig. R2a).
While the one without NNFQSE increased to 7.8 μm, showing more than ten times growth rate of about 123% (Fig. R2b). Therefore, with NNFQSE, the ionic flux will be more slight and the dendrite growth is also perfectly inhibited (Fig. R3).  3. The residue of solid-state products in cathode and the pulverization and dendrite situation of lithium anode after long-term stability tests should be further discussed.

Response:
Thanks for the comments. The characterizations of cathode and lithium anode after long-term cycling have performed by high resolution scanning electron microscope (SEM). As shown in Fig. R4a, the cathode of Li|SiO2-SO3Li/PVDF-HFP|O2 still maintains hollow tubular structure and there only exists little residue of solid-state products after 5000 h cycling. In contrast, the cathode structure of control groups becomes deformation or even collapsed, and there also filled with residue, which hinders the persistent cycling (Fig. R4b-c). On the anode side, the lithium anode surface of Li| SiO2-SO3Li/PVDF-HFP|O2 exhibits bright with metallic luster (Fig. R5a), and its SEM morphology shows smooth without obvious dendrite growth (Fig. R5d-e). While the anode of Li|SiO2/PVDF-HFP |O2 turns black (Fig. R5b) and appears rough surface (Fig. R5f-g). Furthermore, as displayed in Fig.   R5c, the lithium anode of Li|PVDF-HFP|O2 even turns pulverized. These results further confirm that the NNFQSE SiO2-SO3Li/PVDF-HFP can not only trap the liquid electrolyte inside to prevent the side reaction with cathode and reduce the residue of solid-state products, but also suppress lithium dendrite growth.  In this work, the authors reported that using SiO2-SO3Li/PVDF-HFP as non-Newtonian fluid quasi-solid electrolyte in Li-O2 battery can effectively eliminate dendrite issues during cycling measurements along with good liquid electrolyte retention and Li + conductivity. As the results, the battery can perform excellent cycling stability of 5000 hours at 100 mA g -1 .
Response: We thank the reviewer for the positive comments on the impact of our work. We have revised the manuscript and answered questions following your comments. Some issues are addressed as follows: 1. It is really hard to see that the S are uniformly distributed on SiO2-SO3Li particles from EDS-mapping result in Figure S3. Please check the result and give more clear demonstration.
Response: Thanks for the comments. We have carried out more clear demonstration by elemental mapping of TEM. The images in Fig. R6 can further verify the S are uniformly distributed on SiO2-SO3Li particles. 2.The FTIR results should give more clarifications between the SiO2, SiO2-SO3H and SiO2-SO3Li to prove the varied surface chemistry. It is claimed by author that the peak of Si-OH in 2347 cm-1 disappears in SiO2-SO3Li compared to SiO2 due to sulfonated treatment, but why there is a peak of Si-OH at 1632 cm-1 in both SiO2 and SiO2-SO3Li samples in Figure 2d and FigureS5? What the difference between these vibration features?
Response: Thanks for the valuable suggestion. As shown in Fig. R7, we have retested the FTIR spectra of SiO2. The peak in 2347 cm -1 also disappears, which should be corresponded to CO2. Moreover, the peak at 1632 cm -1 may corresponded to the bending vibration of -OH in H2O. In addition, the region of 1000-1300 cm −1 corresponding to Si-O-Si stretching adsorption turns widen both in SiO2-SO3H and SiO2-SO3Li, and the characteristic peak of -SO3 − stretching at 1190 cm −1 indicates the sulfonated treatment. Updates to the revised manuscript: We have added these details in the revised manuscript on Page 7 (Line 125-131, highlighted in yellow) and corrected on Supplementary Fig. 6. 3.Please explain why the core level XPS spectra of S 2p seems very poor signal but showed much stronger intensity in survey spectra in Figure 2e.
Response: Thanks for pointing out the mistake. We have carried out another XPS analysis of SiO2-SO3H to ensure the the position of sulfur (Fig. R8b). After adding the fillers into polymer, there was still a peak at the position, but the signal is weakened. The result was corrected the S 2p signal in XPS survey spectra in Fig. R8 and revised in Fig. 2e.

Response: Thanks for the valuable suggestion. Mercury intrusion porosimetry (MIP) measurements
were carried out to characterize porous features. As shown in Table R1, PVDF-HFP is observed with the highest porosity ratio. After adding SiO2 nanofillers into PVDF-HFP, the porosity ratio is decreased from 74.6% to 58.4%, and the volume density is increased from 0.34 g mL -1 to 0.59 g mL -1 , which further proves that the pores of PVDF-HFP are blocked by unmodified SiO2, corresponding with the SEM observations ( Supplementary Fig. 2c-d). For SiO2-SO3Li/PVDF-HFP, the porosity ratio, volume density, apparent (skeleton) density and total pore volume all fall in between PVDF-HFP and SiO2/PVDF-HFP, revealing that the sulfonate fillers SiO2-SO3Li can make the skeleton matrix maintain certain porosity without being blocked. In addition, it is noticed that the total pore area increases from 7.56 m 2 g −1 , 16.06 m 2 g −1 to 18.23 m 2 g −1 , respectively, owing to the wrinkled radial structure and beauteous folded spherical morphology of SiO2 nanofillers (Supplementary Fig. 3) and the surface channels of SiO2-SO3Li have become deeper and richer after being modified (Fig. 2b-c  Updates to the revised manuscript: We have added these details in the revised manuscript on Page 6 (Line 100-116, highlighted in yellow) and supplementary Table1.
5. Please explain why the stable electrochemical window of SiO2-SO3Li/PVDF-HFP is much higher than the others? Besides, LSV with same scan rate to low potential area should also provide to examine its reduction behavior.  (Adv. Mater. 2019, 31, 1902029；Adv. Mater. 2022) 7 Li-NMR spectrum was performed to prove the interaction. As shown in Fig. R9, SiO2-SO3Li/PVDF-HFP shows the minimum chemical shift. The variation trend is consistent with LSV in Fig. 3i, which indicates the interactions have changed the chemical environment of composite polymer and LE and then tuned the electrochemical window. The chemical down shift means the shielding effect is enhanced and the electron cloud density around lithium nucleus is increased. The lithium nucleus in SiO2-SO3Li/PVDF-HFP has the largest electron cloud density. That means the environment (such as anoins, solvent molecules, et al.) around the lithium nucleus affords extra electrons to Li + and the cloud density of the environment except Li + is reduced, which enhances the oxidation potential. This may lead to the highest oxidation potential of SiO2-SO3Li/PVDF-HFP. In addition, the interactions make more liquid electrolyte anchor inside the skeleton of SiO2-SO3Li/PVDF-HFP and reduce the contact with electrode. So, the oxidation decomposition and parasitic reaction of liquid electrolyte are also inhibited to some extent.

Response
To examine its reduction behavior, another LSV test of was conducted from 3.5 V to 0 V with the same scan rate 1 mV s -1 (Fig. R10). PVDF-HFP exhibits three peaks at ~1.52 V, ~1.17 V and ~0.77 V, which are indexed as the LiTFSI and TEGDME decomposition peaks of liquid electrolyte, respectively, as reported before. (Angew. Chem. Int. Ed. 2022, e202115909) Therein, the 1.52 V peak accounts for the initial dissociation of C-F or S-F bonds, and the 1.

Response:
We thank the reviewer for the positive comments on the impact of our work. We have revised the manuscript and answered questions following your comments.
1. The author calculated the contact angle of SiO2-SO3Li/PVDF-HFP with a different tangent center from control samples (Fig. S7). A wrong conclusion about wettability was obtained. From the direct view, the contact angles of the three samples are similar. The enhanced wettability of SiO2-SO3Li/PVDF-HFP is doubting.
Response: Thanks for pointing out the mistake. I was careless to mix up the images of contact angle when typeseted. We have switched back the original data and revised in Supplementary Fig. 8.

The order of Figures in supporting information is inconsistent with the manuscript.
Response: Thanks for pointing out and comment. We have corrected the orders.
3. The preparation procedure of the cathode should be illustrated in the experimental section.
Response: Thanks for the suggestion. We have added the related preparation procedure of the cathode in the experimental section (Manuscript Page 24, Line 458-464).
4. From presented photos of SiO2-SO3Li/PVDF-HFP before and after cycling, the cycled electrode became thicker than the initial, and why?
Response: Thanks for the comment. Before cycling, the SiO2-SO3Li/PVDF-HFP was thick as 100 μm (Supplementary Fig. 1a-b). After full swelling, its thickness increased to 112 μm. Coupling with the shearing action during plating/stripping process, SiO2-SO3Li/PVDF-HFP may turn softer and change to elastic jelly ( Supplementary Fig. 17a), which result in the final 116 μm thick after cycling.
5. From a recent literature review of LOBs (doi.org/10.1039/D2CS00003B), the battery chemistry is significantly affected by the electrolyte. How the SiO2-SO3Li/PVDF-HFP electrolyte affects the growth and decomposition of discharge products is yet lacking, as well as the crystal structure and morphology.
Response: Thanks for the valuable comments. In order to better understand how the SiO2-SO3Li/PVDF-HFP electrolyte affects the growth and decomposition of discharge products, we investigated the morphology and crystal structure of the cathode after discharge and charge. As shown in Fig. R12, high-magnification SEM images reveal that the discharge products of Li2O2 are formed on the cathode (Fig. R12a-b) and completely disappeared after charging ( Fig. R12c-d), demonstrating good reversibility during the charge/discharge process. Moreover, X-ray diffraction (XRD) analyses of the pristine, discharged, and charged cathodes were carried out in Fig. R12e.
Due to the hollow tubular structure of cathode, the XRD signal is shielded to a certain extent. The discharged cathode exhibits two weak peaks located at 32.9° and 35.8°, which correspond to the (100) and (101)

Updates to the revised manuscript and supplementary information:
We have added these results in Supplementary Fig. 23 and Supplementary Note 2, details in the revised manuscript on Page 21 (Line 396-397), highlighted in yellow.
6. Although SiO2-SO3Li/PVDF-HFP exhibited a wide electrochemical window, reactive oxygen species generated during the discharging and charging of LOBs could also induce the decomposition of the electrolyte and functional groups, especially during ultra-long operation. Relevant discussions and analyses are recommended in order to obtain a more comprehensive understanding of the stability of the designed electrolyte.